Conventionally, solid oxide fuel cells (SOFC) are constituted so that in the startup step, multiple steps for reforming fuel in the reformer are passed through, being the partial oxidation reforming reaction step (the POX step), the auto-thermal reforming reaction step (the ATR step), and the steam reforming reaction step (the SR step); there is then a transition to electrical power generation (see Patent Document 1, for example).
In an SOFC, by executing these steps in sequence, the temperature of the reformer, fuel cell stack, and the like disposed within a fuel cell module housing chamber can be raised to an operating temperature. I.e., at startup of the solid oxide fuel cell, once the reformer is heated by combustion of fuel and the reformer temperature rises somewhat, the partial oxidation reforming reaction (POX step), which is an exothermic reaction, is generated by the reaction of fuel and reforming air in the reformer. Next, when the reformer temperature rises further due to the fuel combustion heat and reaction heat from the partial oxidation reforming reaction, water is introduced into the reformer, and the endothermic steam reforming reaction (the ATR step) is occurring in addition to the partial oxidation reforming reaction. Also, when the reformer temperature rises, the supply of air for reforming is stopped, and only the steam reforming reaction (the SR step) occurs inside the reformer. Electricity is generated in the fuel cell stack using hydrogen produced by this steam reforming reaction.    Patent Document 1: JP 2004-319420 A